The development of optical switching equipment along with the use of dense wavelength division multiplexing (DWDM) has led many service providers to replace optical ring networks based on SONET (Synchronous Optical Networking) and SDH (Synchronous Digital Hierarchy) protocols with optical mesh networks. The large bandwidth capacity of optical networks makes them ideal for high speed data applications and the use of DWDM allows aggregation of many channels onto a single fiber. Consequently, a single failure within the network can result in a large amount of data loss and may impact many users. Because operators typically provide guaranteed quality of service (QoS) requirements in service level agreements, the resilience of the optical network is becoming increasingly important to network operators.
Resilience in an optical network can be achieved by adding protection resources to carry back-up traffic in addition to resources for the primary traffic. Such protection schemes may be generally categorized as single-path protection schemes and multi-path protection schemes.
In single-path protection schemes, the primary traffic is routed over a single path from a source node to a destination node, and back-up traffic is routed over a different path. The back-up path should not share any links with the primary path. For complete protection, commonly referred to as 1+1 protection, the amount of resources allocated for the back-up traffic should be equal to the resources allocated for the primary traffic. Consequently, single-path protection providing 1+1 protection results in a 100% resource overbuild.
Multi-path protection schemes provide an attractive alternative to single-path protection schemes. In multi-path protection schemes, the primary traffic is routed over multiple paths from the source node to the destination node, and a single back-up path can provide protection for multiple primary paths. For example, if the primary traffic is equally split between two link-disjoint paths, 100% protection against a single failure can be provided by a single back-up path capable of handling 50% of the primary traffic. Multi-path protection has some advantages over single-path protection. These advantages include: 1) less constraint on bandwidth availability; 2) lower resource overbuild than single-path protection; 3) increased reliability; and 4) less constraint on transmission impairment.
Multi-path provisioning and protection is enabled by Virtual Concatenation (VCAT) and Link Capacity Adjustment Scheme (LCAS) in SONET/SDH networks. VCAT is an inverse multiplexing technique that groups an arbitrary number of SONET/SDH containers to create a larger container, thus providing flexible and efficient bandwidth allocation. Network operators can combine any number of low-order or high-order containers depending on the switching granularity to create a VCAT group. A connection carried by a VCAT group can be split into multiple paths at the source node and merged at the destination node as each group member is routed independently. The capacity of a VCAT group can be dynamically and hitlessly changed, i.e., increased and decreased, by the LCAS protocol. LCAS also provides survivability capabilities by automatically decreasing the capacity if a member of the VCAT group experiences a failure in the network, and increasing the capacity when the network fault is repaired.
Because VCAT/LCAS offers multi-path protection in the SONET/SDH layer, it requires all nodes to be configured to implement SONET/SDH protocols. Further, intermediate processing requires digital processing, which results in a high processing cost, high power consumption per unit traffic, and complex control and management algorithms. Another drawback is that VCAT/LCAS requires granular traffic in small containers so that traffic can be merged and split flexibly at intermediate nodes.
Optical provisioning and protection may also be provided in the optical layer. Multi-path provisioning in the optical layer has some advantages over multi-path provisioning at the SONET/SDH layer. Multi-path provisioning at the optical layer requires fewer processing resources and lower signaling overhead. Further, multi-path provisioning at the optical layer does not require granular traffic so there is more flexibility in allocating resources. However, multi-path provisioning in the optical layer, e.g., Split Spectrum, sets up wavelength/spectrum channels that physically bind bandwidth to each sub-signal. These sub-signals cannot be split at intermediate nodes. This property is referred to as the integrality of sub-signals. Thus, protection schemes for multi-path protection in the optical layer must be realized on a sub-signal basis.